Semiconductor memory devices may be classified into volatile memory devices and nonvolatile memory devices. In contrast to volatile memory devices, nonvolatile memory devices, such as flash memory devices, retain stored data even when power is removed. Therefore, nonvolatile memory devices, such as flash memory devices, are widely used in memory cards and in electronic devices such as mobile communication terminals.
Flash memory devices have been used in a wide range of electronic applications, such as portable computers, personal digital assistants (PDAs), digital cameras, portable music players, and cellular telephones. A conventional flash memory has a memory array that includes a large number of memory cells arranged in row and column fashion. Each of the memory cells includes a floating gate field-effect transistor configured to hold a charge. The memory cells are usually grouped into blocks. Each of the cells within a block can be electrically programmed in a random basis by charging the floating gate. The charge can be removed from the floating gate by a block erase operation. The data in a cell is determined by the presence or absence of the charge in the floating gate. Flash memory devices may be classified as NAND type and NOR type devices according to the structure of their cell arrays. In NOR flash devices, a column of memory cells are coupled in parallel with each memory cell coupled to a bit line. In NAND flash devices, a column of memory cells are coupled in series with only the first memory cell of the column coupled to a bit line.
Due to rapidly growing digital information technology, there are demands to continuingly increase the memory density of the flash memory devices while maintaining, if not reducing, the size of the devices. Three dimensional (3D)-NAND flash memory devices have been investigated for increasing the memory density.
Fabrication of a conventional 3D-NAND flash memory device requires creating high aspect ratio openings (e.g., an aspect ratio of at least 20:1) in a stack of alternating insulating materials and conductive materials on a substrate. The openings are formed by etching the stack of alternating insulating materials and conductive materials. To prevent etching of the substrate, an etch stop material is present between the substrate and the stack of alternating materials. However, selecting a material as the etch stop that meets stringent wet clean selectivity requirements is a challenge. When amorphous aluminum oxide is used as the etch stop material, recesses formed in the amorphous aluminum oxide may become filled with polysilicon during later processing acts. The undesirable residual polysilicon in these recesses jeopardizes the controllability of the channel characteristics and the reliability of the 3D-NAND flash memory device. Therefore, it would be beneficial to have an etch stop material that meets stringent wet clean selectivity requirements and minimizes, if not eliminates, the formation of the residual polysilicon in the recesses in the amorphous aluminum oxide.